I'm definitely not a DIY electronics guy but I did get my RW connected to my VP2+ recently as well, with many thanks to CW2274 and Valentine Wx for the great info. Connecting directly to the existing RW wire nuts made for an easy task. But I seem to recall having issues until I remembered to disconnect the RW counter. Can't tell in the pic if the counter is connected (and powered), so not sure if this could be causing the erroneous rain data. Otherwise, aside from a possible short already mentioned, maybe it was bouncing in the wind?
In any case, hope you find a solution. I'm very happy having replaced my original tipping bucket with the RW one.